diff --git a/CHANGELOG b/CHANGELOG
index bf725919588..1abd7af611a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -8,6 +8,7 @@ CHANGELOG Roundcube Webmail
 - Fix rsync error handling in installto.sh script (#5562)
 - Fix some advanced search issues with multiple addressbooks (#5572)
 - Fix so group/addressbook selection is retained on page refresh
+- Fix bug where image data URIs in css style were treated as evil/remote in mail preview (#5580)
 
 RELEASE 1.2.3
 -------------
diff --git a/program/lib/Roundcube/rcube_utils.php b/program/lib/Roundcube/rcube_utils.php
index ba750b08034..c0d109acdab 100644
--- a/program/lib/Roundcube/rcube_utils.php
+++ b/program/lib/Roundcube/rcube_utils.php
@@ -396,7 +396,7 @@ public static function mod_css_styles($source, $container_id, $allow_remote = fa
         // ignore the whole block if evil styles are detected
         $source   = self::xss_entity_decode($source);
         $stripped = preg_replace('/[^a-z\(:;]/i', '', $source);
-        $evilexpr = 'expression|behavior|javascript:|import[^a]' . (!$allow_remote ? '|url\(' : '');
+        $evilexpr = 'expression|behavior|javascript:|import[^a]' . (!$allow_remote ? '|url\((?!data:image)' : '');
 
         if (preg_match("/$evilexpr/i", $stripped)) {
             return '/* evil! */';
diff --git a/program/steps/mail/func.inc b/program/steps/mail/func.inc
index d37825a8ab3..0e13c845382 100644
--- a/program/steps/mail/func.inc
+++ b/program/steps/mail/func.inc
@@ -969,7 +969,7 @@ function rcmail_washtml_callback($tagname, $attrib, $content, $washtml)
 
         // now check for evil strings like expression, behavior or url()
         if (!preg_match('/expression|behavior|javascript:|import[^a]/i', $stripped)) {
-            if (!$washtml->get_config('allow_remote') && stripos($stripped, 'url(')) {
+            if (!$washtml->get_config('allow_remote') && preg_match('/url\((?!data:image)/', $stripped)) {
                 $washtml->extlinks = true;
             }
             else {
diff --git a/tests/Framework/Utils.php b/tests/Framework/Utils.php
index 5f70544d467..ba04e15454d 100644
--- a/tests/Framework/Utils.php
+++ b/tests/Framework/Utils.php
@@ -214,6 +214,10 @@ function test_mod_css_styles_xss()
 
         $mod = rcube_utils::mod_css_styles(".test { position:/**/fixed; }", 'rcmbody');
         $this->assertEquals("#rcmbody .test { position: absolute; }", $mod, "Replace position:fixed with position:absolute (2)");
+
+        // allow data URIs with images (#5580)
+        $mod = rcube_utils::mod_css_styles("body { background-image: url(data:image/png;base64,123); }", 'rcmbody');
+        $this->assertEquals("#rcmbody { background-image: url(data:image/png;base64,123); }", $mod, "Data URIs in url() allowed");
     }
 
     /**